There exists significant activity in the development of digital x-ray image data capture systems. In such systems direct conversion to an electrical signal of the incident radiation is obtained using a plurality of sensors in an array. The sensor output is almost invariably immediately converted to a digital signal and further processed and stored in a databank for use in the eventual display of the data as a radiogram. U.S. Pat. No. 5,313,066 issued to Lee et al. and U.S. Pat. No. 5,315,101 issued to Hughes et al describe typical such sensor arrays and their contents are incorporated herein by reference. Even though several different technologies are being utilized, the output data is quite similar.
A major advantage of digital data detection systems is the wide dynamic range of signal capture. Display media, such as radiographic film or Cathode Ray Tube (CRT) displays, on the other hand, have a substantially more limited dynamic range. A typical digital x-ray capture system can have a useful dynamic detection range of greater than a 1,000:1. However, the useful image data is generally limited to a dynamic range of less than 100:1. There is, therefore, need to determine and select the optimal limited range of useful data for diagnostic display, and then properly display such range on the available display medium.
This problem which reduces to a need for a method whereby the exposure sensor output is mapped onto the display device density transfer function has been addressed by the art in numerous ways. Typically the sensor output is digitized and a histogram of the frequency of occurrence of digital values representing detected exposure is constructed. Following construction of the histogram, cutoff points eliminating values under selected minimum occurrence for both ends of the scale are determined and the digital values in the remaining range are mapped onto the display transfer function. These steps are rather fundamental and intuitive. What is significant and the subject of continuing research is the specific selection process for the two cutoff points and the manner in which the remaining density values are actually mapped on the transfer function.
U.S. Pat. No. 5,164,993 issued Nov. 17, 1992 to Capozzi et al. together with U.S. Pat. Nos. 5,046,118 issued to Ajewole et al. and 4,868,651 issued to Chou et al. are believed to represent the current state of the art in explaining and solving the problems associated with such displays.
The currently available solutions do not adequately address the problem of adapting the digital value mapping to a particular type of both patient and examination. For example, a different portion of the data generated by the radiation sensors is of interest in displaying a radiogram depending on whether the radiogram is one of an extremity or a chest cavity, whether the patient is thin or obese, and what is the area desired to be displayed with maximum diagnostic efficiency. There is, therefore, still need for a system which addresses these problems and which with simple operator input automatically maps the available data in a way as to optimize the display medium density range for a particular set of examination type, patient characteristics and display medium capabilities.
It is an object of this invention to provide a method for automatically identifying the range of useful digital values to be used for diagnostic display, and to provide an appropriate gray scale transfer to optimize the diagnostic value of the final displayed image, either hard or soft copy.